Various seat constructions of office chairs, customarily known as swivel chairs, include an inclined support structure which extends at an angle from a central support post, and uses a torsion spring between the support structure and the seat itself, the torsion spring being arranged to bias the seat into an upper tilted position. The spring tension of the torsion spring can usually be changed and a blocking arrangement between the support and the seat is provided in order to lock the seat in position in at least two different tilted angles. Chairs of this type have been commercially available for about two decades. The support extending from the central post of these known chairs is relatively short and, hence, if the seat is deflected downwardly, the forward portion of the seat will raise up. If the user is not very tall, or the support post is at a high level, the user may lose floor contact with his feet. This is frequently found undesirable by users, since a sense of instability is conveyed by loss of floor contact.
The chairs of this construction have another disadvantage, in that the blocking device has only a lower blocking position and an upper blocking position, so that the chair cannot be locked in desired intermediate positions. The tension of the torsion spring can be changed by the user in depedence of the weight of the user. Usually, however, adjustment screws for such a device are located far below the seat and inaccessible while the user is actually using the seat. Thus, the adjustment screw can be adjusted only when the user is off the chair, then the chair must be tried, the screw again re-adjusted, and so on. This continued on-off and trial-by-error adjustment is awkward and time-consuming.
The referenced U.S. Pat. No. 4,832,402, Zund, describes a chair which has an attachment plate to which a seat structure can be secured. The attachment plate can be tilted about a horizontal tilt axis at the forward side thereof, and is tiltably secured to an upwardly inclined support structure. A gas spring extends between the support structure and the carrier plate, projecting at an approximately right angle from the support structure. This gas spring operates as a spring which has the tendency to move the attachment plate from a lower tilt position to an upper tilt position, and, additionally, to function as a blocking arrangement to block the attachment plate and the seat secured thereto in any desired tilt position, in accordance with an adjustment made by the user. A spiral spring is located coaxially to the gas spring, and the bias of the spiral spring can be adjusted by a double-arm lever, controlled by a wing nut, in accordance with the weight of the user. The wing nut is located beneath the seat, and thus adjustment thereof cannot be carried out while the user is seated on the chair.
The chair of this construction, in spite of the inaccessibility of the adjustment for the spiral spring, does not present a clean appearance since both the gas spring as well as the spiral spring are readily visible from the side. This is unsatisfactory from an aesthetic point of view and this disadvantage has been found particularly annoying when office equipment is considered which should not only be functional but also of pleasing appearance design.
The chairs of the prior art, in general, have a disadvantage with respect to maintenance or repair. In actual practice, the elements of chairs which wear out, besides the upholstery, are the adjustment mechanism. The adjustment mechanisms of many modern chairs operate for years without any service problems; yet, from time to time an element such as the gas spring or other movable parts require replacement or joints have to be oiled or greased. The inaccessibility of replacement parts and of moving connections or joints, which should be maintained, contribute to lack of proper maintenance.